Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Losses
Less than 12 months	$ 917	$ 21,218
More than 12 months	22,776	14,254
Total	23,693	35,472
Fair values
Less than 12 months	70,643	432,053
More than 12 months	438,304	146,148
Total	508,947	578,201
Municipal
Losses
Less than 12 months	50	262
More than 12 months	120	10
Total	170	272
Fair values
Less than 12 months	13,022	27,491
More than 12 months	8,383	67
Total	21,405	27,558
Corporate
Losses
Less than 12 months	68	12,935
More than 12 months	11,878	5,600
Total	11,946	18,535
Fair values
Less than 12 months	4,808	193,239
More than 12 months	208,971	44,342
Total	213,779	237,581
Foreign
Losses
Less than 12 months	472	7,608
More than 12 months	10,625	8,604
Total	11,097	16,212
Fair values
Less than 12 months	31,918	186,221
More than 12 months	216,135	101,294
Total	248,053	287,515
U.S. Treasury Bonds
Losses
Less than 12 months	327	413
More than 12 months	153	40
Total	480	453
Fair values
Less than 12 months	20,895	25,102
More than 12 months	4,815	445
Total	$ 25,710	$ 25,547